U.S. patent application number 13/810765 was filed with the patent office on 2013-05-09 for formable aluminum alloy sheet.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is Mitsuhiro Abe, Makoto Morishita. Invention is credited to Mitsuhiro Abe, Makoto Morishita.
Application Number | 20130112323 13/810765 |
Document ID | / |
Family ID | 45723463 |
Filed Date | 2013-05-09 |
United States Patent
Application |
20130112323 |
Kind Code |
A1 |
Abe; Mitsuhiro ; et
al. |
May 9, 2013 |
FORMABLE ALUMINUM ALLOY SHEET
Abstract
The present invention provides an aluminum alloy sheet for
forming which is a high-Mg-content Al--Mg alloy sheet reduced in
.beta.-phase precipitation and improved in press formability. This
aluminum alloy sheet for forming comprises an Al--Mg alloy
containing 6.0-15.0 mass % Mg. In each of square regions, each side
of which has the dimension of the whole sheet width (W), that are
set in a surface of the alloy sheet, the concentration of Mg is
measured at width-direction measurement points, Px, set at given
intervals a and b respectively in the sheet-width direction and the
sheet-length direction, and the average of the values of Mg
concentration measured at the plurality of width-direction
measurement points (Px) is taken as a width-direction average Mg
concentration (Co). The concentration of Mg is measured at a
plurality of thickness-direction measurement points (Py) set at a
given interval in the sheet-thickness direction throughout the
whole sheet thickness with respect to the plurality of
width-direction measurement points (Px), and the average of the
values of Mg concentration measured at the plurality of
thickness-direction measurement points (Py) is taken as a
thickness-direction average Mg concentration (Ci). The absolute
value of the degree of regional Mg segregation (X) defined by the
difference (Ci-Co) between the thickness-direction average Mg
concentration (Ci) and the width-direction average Mg concentration
(Co) is 0.5 mass % or less at most and is 0.1 mass % or less on
average.
Inventors: |
Abe; Mitsuhiro; (Moka-shi,
JP) ; Morishita; Makoto; (Moka-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Abe; Mitsuhiro
Morishita; Makoto |
Moka-shi
Moka-shi |
|
JP
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Hyogo
JP
|
Family ID: |
45723463 |
Appl. No.: |
13/810765 |
Filed: |
August 23, 2011 |
PCT Filed: |
August 23, 2011 |
PCT NO: |
PCT/JP2011/068979 |
371 Date: |
January 17, 2013 |
Current U.S.
Class: |
148/551 ;
148/439; 148/440 |
Current CPC
Class: |
C22C 21/08 20130101;
C22F 1/047 20130101; B22D 11/055 20130101; B22D 11/003 20130101;
B22D 11/059 20130101; B22D 11/0622 20130101; B21B 2003/001
20130101; C22C 21/06 20130101 |
Class at
Publication: |
148/551 ;
148/440; 148/439 |
International
Class: |
C22F 1/047 20060101
C22F001/047; C22C 21/08 20060101 C22C021/08; C22C 21/06 20060101
C22C021/06 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 25, 2010 |
JP |
2010-187756 |
Claims
1. An aluminum alloy sheet, comprising Mg in a content of 6.0
percent by mass or more and 15.0 percent by mass or less and
further comprising Al and an impurity, the aluminum alloy sheet
having: a width-direction average Mg concentration of Co, wherein
the width-direction average Mg concentration, Co, is an average of
Mg concentrations measured at a plurality of width-direction
measurement points, wherein the plurality of width-direction
measurement points is arranged at predetermined spacings in a width
direction and in a length direction of the sheet, respectively, in
a square region set on a surface of the aluminum alloy sheet,
wherein each side of the square region has a dimension of an entire
sheet width; a thickness-direction average Mg concentration, Ci,
for each of the plurality of width-direction measurement points,
wherein each Ci is an average of Mg concentrations measured at a
plurality of thickness-direction measurement points arranged in a
thickness direction of the sheet at a predetermined spacing
throughout an entire sheet thickness; and regional Mg segregation
degrees of X each defined as a difference, Ci-Co, between each
thickness-direction average Mg concentration, Ci, and the
width-direction average Mg concentration, wherein absolute values
of the regional Mg segregation degrees, X, have a maximum of 0.5
percent by mass or less and an average of 0.1 percent by mass or
less.
2. The aluminum alloy sheet of claim 1, wherein the aluminum alloy
sheet has thickness-direction Mg concentrations, Ct, wherein the
thickness-direction Mg concentrations are measured in a thickness
direction of the sheet at a predetermined spacing throughout the
entire sheet thickness for the width-direction measurement points
upon determination of the regional Mg segregation degrees, X; and
thickness-direction Mg segregation degrees, Y, each defined as a
difference, Ct-Ci, between each thickness-direction Mg
concentration, Ct, and the thickness-direction average Mg
concentration, Ci, corresponding to the respective width-direction
measurement point, in which absolute values of the
thickness-direction Mg segregation degrees, Y, have a maximum of 4
percent by mass or less and an average of 0.8 percent by mass or
less.
3. The aluminum alloy sheet of claim 1, wherein Mg is in a content
of more than 8 percent by mass and less than or equal to 14 percent
by mass.
4. The aluminum alloy sheet of claim 1, wherein the impurity
comprises at least one element selected from the group consisting
of Fe in a content of 1.0 percent by mass or less, Si in a content
of 0.5 percent by mass or less, Ti in a content of 0.1 percent by
mass or less, B in a content of 0.05 percent by mass or less, Mn in
a content of 0.3 percent by mass or less, Cr in a content of 0.3
percent by mass or less, Zr in a content of 0.3 percent by mass or
less, V in a content of 0.3 percent by mass or less, Cu in a
content of 1.0 percent by mass or less, and Zn in a content of 1.0
percent by mass or less.
5. The aluminum alloy sheet of claim 2, wherein Mg is in a content
of more than 8 percent by mass and less than or equal to 14 percent
by mass.
6. The aluminum alloy sheet of claim 2, wherein the impurity
comprises at least one element selected from the group consisting
of Fe in a content of 1.0 percent by mass or less, Si in a content
of 0.5 percent by mass or less, Ti in a content of 0.1 percent by
mass or less, B in a content of 0.05 percent by mass or less, Mn in
a content of 0.3 percent by mass or less, Cr in a content of 0.3
percent by mass or less, Zr in a content of 0.3 percent by mass or
less, V in a content of 0.3 percent by mass or less, Cu in a
content of 1.0 percent by mass or less, and Zn in a content of 1.0
percent by mass or less.
7. The aluminum alloy sheet of claim 1, comprising a fine
grain.
8. The aluminum alloy sheet of claim 7, having an average grain
size of 20 .mu.m or more and 100 .mu.m or less on the surface.
9. A method of producing the aluminum alloy sheet of claim 1, the
method comprising: melt-casting by melting a high-Mg-content Al--Mg
alloy to obtain a molten metal, and preparing a cast strip from the
molten metal by continuous strip casting; soaking the cast strip in
a continuous heat-treating furnace at a temperature of 400.degree.
C. or above and a liquidus temperature or below; cold rolling the
cast strip to convert the cast strip into a deformation structure
to obtain a cold-rolled sheet; and final annealing the cold-rolled
sheet at a temperature of 400.degree. C. or above and below the
liquidus temperature.
10. The method of claim 9, wherein the continuous strip casting
comprises a fixed graphite mold.
11. The method of claim 10, wherein the continuous strip casting
comprises: pouring a molten metal stored in a holding furnace
through an inlet into the fixed graphite mold; and solidifying the
molten metal in the fixed graphite mold, by cooling with a
water-cooling jacket.
12. The method of claim 11, wherein the cooling is performed at a
rate of 15.degree. C./s wherein the cast strip has a thickness of
from 5 to 20 mm.
13. The method of claim 12, wherein the pouring is performed at a
temperature higher than the liquidus temperature by 50.degree. C.
or more and 250.degree. C. or less.
14. The method of claim 10, wherein the continuous casting process
is performed at an average casting rate of 100 mm/min or more and
500 mm/min or less.
15. The method of claim 10, wherein the continuous casting
comprises facing in wherein the cast strip is cut or shaved.
16 The method of claim 9, wherein the soaking is performed for a
duration of one second or shorter.
17. The method of claim 9, wherein the cold-rolling comprises
cold-rolling the cast strip to a thickness of a product sheet of
0.1 mm or more and 13 mm or less.
18. The method of claim 17, further comprising a process annealing
performed midway through the cold rolling to obtain a cold rolling
reduction in a final cold rolling of 60% or less.
19. The method of claim 9, wherein the final annealing is performed
at a temperature of 450.degree. C. or above and below the liquidus
temperature.
20. The method of claim 9, further comprising cooling after final
annealing at a cooling rate of 10.degree. C./s or more at a
temperature of 500.degree. C. down to 300.degree. C.
Description
TECHNICAL FIELD
[0001] The present invention relates to a formable aluminum alloy
sheet which is an Al--Mg alloy sheet containing Mg in a high
content and has satisfactory formability.
BACKGROUND ART
[0002] As is well known, a variety of aluminum alloy sheets has
been generally widely used in transportation machines such as
automobiles, ships, aircraft, and vehicles; machines; electric
products; construction materials; structures; optical appliances;
and members or parts of wares, according to properties of
respective alloy categories. Such aluminum alloy sheets often
formed typically through stamping into the members and parts for
use in these applications. Of aluminum alloys, Al--Mg alloys being
in good balance between strength and ductility are advantageous for
satisfactory formability. The Al--Mg alloys are represented by
alloys prescribed in Japanese Industrial Standards (JIS) A 5052 and
A 5182. These Al--Mg alloy sheets, however, have inferior ductility
and thereby have inferior formability to customary cold-rolled
steel sheets. As solutions to these disadvantages, chemical
compositions of Al--Mg alloy sheets and optimization of
manufacturing conditions of them have been studied.
[0003] Typically, an Al--Mg alloy, when having an increased Mg
content of more than 6 percent by mass, preferably more than 8
percent by mass, may have better strength-ductility balance.
However, a sheet of such a high-Mg-content Al--Mg alloy is hardly
industrially manufactured by a common manufacturing method of
casting a material into an ingot typically through direct chill
(DC) casting, soaking the ingot, and hot-rolling the soaked ingot.
This is because Mg is segregated in the ingot upon casting and
causes the Al--Mg alloy to have significantly reduced ductility and
to be liable to suffer from cracking upon common hot rolling
process.
[0004] A possible solution to avoid the temperature range at which
cracking occurs is hot rolling at a low temperature. However, it is
also difficult to manufacture the high-Mg-content Al--Mg alloy
sheet through such low-temperature hot rolling. This is because the
high-Mg-content Al--Mg alloy material has remarkably high
resistance to deformation upon the low-temperature rolling, and
this extremely restricts sizes of products to be manufactured in
view of performance of current rolling mills. Independently, a
technique of adding a third element such as Fe or Si has been
proposed so as to allow an Al--Mg alloy to contain Mg in a higher
content. However, the material Al--Mg alloy, if containing the
third element in a high content, tends to suffer from coarse
intermetallic compounds and causes the aluminum alloy sheet to have
low ductility. Increase in Mg content therefore has a ceiling
according to this technique, and it is difficult to allow an Al--Mg
alloy to contain Mg in a content of more than 8 percent by
mass.
[0005] As possible solutions to these issues, various techniques
have been proposed so as to manufacture high-Mg-content Al--Mg
alloy sheets by a twin-roll continuous casting or another
continuous casting process.
[0006] For example, Patent Literature (PTL) 1 describes an aluminum
alloy sheet for automobiles, which is manufactured by twin-roll
continuous casting and is a high-Mg-content Al--Mg alloy sheet
having a Mg content of 6 to 10 percent by mass, in which Al--Mg
intermetallic compounds have an average size of 10 .mu.m or
less.
[0007] PTL 2 describes an aluminum alloy sheet for automotive body
sheets, which is manufactured by continuous casting and is an
Al--Mg alloy sheet having a Mg content of 2.5 to 8 percent by mass.
The aluminum alloy sheet contains Al--Mg intermetallic compounds
having sizes of 10 .mu.m or more in a number density of
300/mm.sup.2 or less and has an average grain size of 10 to 70
.mu.m.
[0008] PTL 3 describes an Al--Mg alloy sheet which is manufactured
by twin-roll continuous casting and has a Mg content of 8 to 14
percent by mass. When Mg concentrations are measured throughout a
thickness direction of the Al--Mg alloy sheet and averaged to give
an average, absolute values of differences (deviation widths)
between the average Mg concentration and the respective Mg
concentrations have a maximum of 4 percent by mass or less and an
average of 0.8 percent by mass or less. This may inhibit the
precipitation of Al--Mg intermetallic compounds.
CITATION LIST
Patent Literature
[0009] PTL 1: Japanese Unexamined Patent Application Publication
(JP-A) No. H07-252571
[0010] PTL 2: JP-A No. H08-165538
[0011] PTL 3: JP-A No. 2007-77485
SUMMARY OF INVENTION
Technical Problem
[0012] Al--Mg intermetallic compounds precipitated upon casting
often cause fracture upon stamping, as described in PTL1 and PTL 2.
Downsizing of Al--Mg intermetallic compounds (also referred to as
".beta. phase(s)") or reduction in amounts of coarse .beta. phases
is therefore effective for increasing stamping performance of a
high-Mg-content Al--Mg alloy sheet. According to the techniques
described in PTL 1 and PTL 2, Al--Mg intermetallic compounds, which
will precipitate upon casting, are suppressed by performing cooling
at a high rate (at a high casting rate) in the casting step. With
an increasing Mg content in an Al--Mg alloy sheet, however, it
becomes difficult to reduce the .beta. phases to such an extent as
not to adversely affect the stamping performance, when the control
of cooling rate in the casting step is employed alone.
[0013] The technique disclosed in PTL 3 employs control of soaking
and final annealing conditions to reduce the level of Mg
segregation (Mg concentration distribution) throughout a thickness
direction of the sheet, to suppress the precipitation of Al--Mg
intermetallic compounds (.beta. phases) caused by segregation of Mg
(nonuniformity in Mg concentration). However, such a
high-Mg-content Al--Mg alloy sheet manufactured by a customary
twin-roll continuous casting process suffers from Mg segregation
also in a width direction. For this reason, it is disadvantageously
difficult to reduce .beta. phases of the high-Mg-content Al--Mg
alloy sheet to such an extent as not to adversely affect the
stamping performance, when the reduction in level of Mg segregation
throughout the thickness direction is employed alone.
[0014] Accordingly, demands have been made to provide a technique
for suppressing .beta. phases of a high-Mg-content Al--Mg alloy
sheet to such an extent as not to adversely affect the stamping
performance, instead of, or in addition to, the control of cooling
rate in a casting step and/or the suppression of the level of Mg
segregation in the thickness direction by the control of conditions
for soaking and final annealing subsequent to the casting step.
[0015] The present invention has been made to solve such problems,
and an object thereof is to provide a formable aluminum alloy sheet
which is a high-Mg-content Al--Mg alloy sheet, less suffers from
the precipitation of .beta. phases inside thereof, and has better
stamping performance (stamping formability).
Solution to Problem
[0016] To achieve the object, the present invention provides a
formable aluminum alloy sheet which contains Mg in a content of 6.0
percent by mass or more and 15.0 percent by mass or less and
further contains Al and impurities. The aluminum alloy sheet has a
width-direction average Mg concentration of Co, where the
width-direction average Mg concentration (Co) is an average of Mg
concentrations measured at plural width-direction measurement
points arranged at predetermined spacings in a width direction and
in a length direction of the sheet, respectively, in a square
region set on a surface of the formable aluminum alloy sheet, each
side of the square region has the dimension of an entire sheet
width. The aluminum alloy sheet has thickness-direction average Mg
concentrations of Ci for the plural width-direction measurement
points, respectively, where the thickness-direction average Mg
concentrations (Ci) are each an average of Mg concentrations
measured at plural thickness-direction measurement points arranged
in a thickness direction of the sheet at a predetermined spacing
throughout an entire sheet thickness. The aluminum alloy sheet has
regional Mg segregation degrees of X each defined as a difference
(Ci-Co) between each of the thickness-direction average Mg
concentrations (Ci) and the width-direction average Mg
concentration (Co), in which absolute values of the regional Mg
segregation degrees (X) have a maximum of 0.5 percent by mass or
less and an average of 0.1 percent by mass or less.
[0017] The formable aluminum alloy sheet, as having the above
configuration, has regional Mg segregation degrees (X) having a
maximum and an average controlled to predetermined levels or less,
which regional Mg segregation degrees (X) are each defined as a
difference between each of the thickness-direction average Mg
concentrations (Ci) and the width-direction average Mg
concentration (Co). The formable aluminum alloy sheet thereby less
undergoes Mg segregation in the entire sheet, namely, both in the
thickness direction and in the width direction; and less suffers
from the precipitation of .beta. phases inside thereof, and
nonuniform deformation and resulting strain concentration upon
forming.
[0018] In a preferred embodiment of the present invention, the
aluminum alloy sheet has thickness-direction Mg concentrations of
Ct, where the thickness-direction Mg concentrations are measured in
a thickness direction at a predetermined spacing throughout the
entire sheet thickness for at least one of the width-direction
measurement points upon determination of the regional Mg
segregation degrees (X); and the aluminum alloy sheet has, in
addition to the regional Mg segregation degrees (X),
thickness-direction Mg segregation degrees of Y each defined as a
difference (Ct-Ci) between each of the thickness-direction Mg
concentrations (Ct) and a thickness-direction average Mg
concentration (Ci) corresponding to the at least one
width-direction measurement point, in which absolute values of the
thickness-direction Mg segregation degrees (Y) have a maximum of 4
percent by mass or less and an average of 0.8 percent by mass or
less.
[0019] The formable aluminum alloy sheet, when having the above
configuration, has thickness-direction Mg segregation degrees (Y)
having a maximum and an average controlled to predetermined levels
or less, in addition to the regional Mg segregation degrees (X).
The thickness-direction Mg segregation degrees (Y) are each defined
as a difference between each of the thickness-direction Mg
concentrations (Ct) and the thickness-direction average Mg
concentration (Ci). The formable aluminum alloy sheet may thereby
further less undergo Mg segregation and further less suffer from
the precipitation of .beta. phases inside thereof, and nonuniform
deformation and resulting strain concentration upon forming.
[0020] In another preferred embodiment of the present invention,
the formable aluminum alloy sheet contains Mg in a content of more
than 8 percent by mass and less than or equal to 14 percent by
mass.
[0021] The formable aluminum alloy sheet, when having the above
configuration, has a Mg content within the specific range, may
thereby have higher strengths and better ductility and less suffer
from the precipitation of .beta. phases inside thereof.
[0022] In yet another preferred embodiment of the formable aluminum
alloy sheet according to the present invention, the impurities
include at least one element selected from the group consisting of
Fe in a content of 1.0 percent by mass or less, Si in a content of
0.5 percent by mass or less, Ti in a content of 0.1 percent by mass
or less, B in a content of 0.05 percent by mass or less, Mn in a
content of 0.3 percent by mass or less, Cr in a content of 0.3
percent by mass or less, Zr in a content of 0.3 percent by mass or
less, V in a content of 0.3 percent by mass or less, Cu in a
content of 1.0 percent by mass or less, and Zn in a content of 1.0
percent by mass or less.
[0023] The formable aluminum alloy sheet, when having the above
configuration, is controlled on the contents of Fe and Si acting as
impurities, may thereby less suffer from the precipitation of
intermetallic compounds inside thereof, and have better fracture
toughness and better stamping performance. The intermetallic
compounds herein are Al--Mg--(Fe, Si) and other Al--Mg
intermetallic compounds; and other intermetallic compounds than
Al--Mg intermetallic compounds, such as Al--Fe and Al--Si
intermetallic compounds. This aluminum alloy sheet is also
controlled on the contents of Ti, B, Mn, Cr, Zr, V, Cu, and Zn
acting as impurities may be protected from adverse effects on
stamping performance.
Advantageous Effects of Invention
[0024] The formable aluminum alloy sheet according to the present
invention less undergoes Mg segregation, less suffers from the
resulting formation of .beta. phases, and can exhibit superior
stamping performance. The formable aluminum alloy sheet can have
further better stamping performance when having a Mg content
controlled within a narrower range or by containing, in addition to
Mg, at least one element selected from Fe, Si, Ti, B, Mn, Cr, Zr,
V, Cu, and Zn in a controlled content.
BRIEF DESCRIPTION OF DRAWINGS
[0025] [FIG. 1] FIG. 1 depicts plural measurement points for Mg
concentrations to be employed upon determination of Mg segregation
degrees of a formable aluminum alloy sheet according to the present
invention, in which view (a) is a plan view, and view (b) is a
cross-sectional view taken along the line A-A in view (a).
[0026] [FIG. 2] FIG. 2 is a cross-sectional view schematically
illustrating a structure of continuous strip casting equipment for
use in the manufacture of a formable aluminum alloy sheet according
to the present invention.
[0027] [FIG. 3] FIG. 3 is a cross-sectional view schematically
illustrating a continuous strip casting equipment for use in the
manufacture of a formable aluminum alloy sheet according to the
present invention.
[0028] [FIG. 4] FIG. 4 is a graph illustrating calculation results
of regional Mg segregation degrees in a formable aluminum alloy
sheet which satisfies conditions specified in the present
invention.
[0029] [FIG. 5] FIG. 5 is a graph illustrating calculation results
of regional Mg segregation degrees in a formable aluminum alloy
sheet which does not satisfy the conditions specified in the
present invention.
[0030] [FIG. 6] FIG. 6 is a graph illustrating calculation results
of thickness-direction Mg segregation degrees in the formable
aluminum alloy sheet which satisfies the conditions specified in
the present invention.
[0031] [FIG. 7] FIG. 7 is a graph illustrating calculation results
of thickness-direction Mg segregation degrees in the formable
aluminum alloy sheet which does not satisfy the conditions
specified in the present invention.
DESCRIPTION OF EMBODIMENTS
[0032] Formable aluminum alloy sheets according to embodiments of
the present invention will be illustrated in detail below.
[0033] A formable aluminum alloy sheet according to an embodiment
of the present invention (hereinafter also simply referred to as an
"aluminum alloy sheet") is a sheet of an aluminum alloy containing
Mg in a high content and has regional Mg segregation degrees X
controlled to predetermined values or less, which regional Mg
segregation degrees X are each defined by a width-direction average
Mg concentration Co and a thickness-direction average Mg
concentration Ci.
[0034] Initially, a chemical composition of the aluminum alloy
sheet according to the present embodiment will be illustrated on
significance of respective alloy elements and on reasons why the
contents are specified.
[0035] The aluminum alloy sheet according to the present embodiment
includes an aluminum alloy containing Mg in a content of 6.0
percent by mass or more and 15.0 percent by mass or less and
further containing Al and impurities, i.e., a high-Mg-content
Al--Mg alloy. In a preferred embodiment, the aluminum alloy sheet
includes a high-Mg-content Al--Mg alloy containing, as another
element than Mg and as an impurity, at least one element selected
from the group consisting of Fe in a content of 1.0 percent by mass
or less, Si in a content of 0.5 percent by mass or less, Ti in a
content of 0.1 percent by mass or less, B in a content of 0.05
percent by mass or less, Mn in a content of 0.3 percent by mass or
less, Cr in a content of 0.3 percent by mass or less, Zr in a
content of 0.3 percent by mass or less, V in a content of 0.3
percent by mass or less, Cu in a content of 1.0 percent by mass or
less, and Zn in a content of 1.0 percent by mass or less.
[0036] Mg
[0037] Magnesium (Mg) element is an important alloy element to
increase strengths and ductility of the aluminum alloy sheet. An
aluminum alloy sheet containing Mg in a content of less than 6.0
percent by mass may have insufficient strengths and ductility and
may fail to exhibit characteristics as a high-Mg-content Al--Mg
alloy, resulting in insufficient stamping performance. In contrast,
an aluminum alloy sheet containing Mg in a content of more than
15.0 percent by mass may be difficult to have Mg segregation, i.e.,
the regional Mg segregation degrees, controlled within the
predetermined range even when its manufacturing method and
conditions are controlled. An aluminum alloy sheet having regional
Mg segregation degrees not controlled within the predetermined
ranges may suffer from precipitation of larger amounts of .beta.
phases, thereby have remarkably inferior stamping performance,
undergo greater work hardening, and thereby have insufficient cold
rolling properties. To avoid these, the aluminum alloy sheet has a
Mg content of 6.0 percent by mass or more and 15.0 percent by mass
or less, and preferably more than 8 percent by mass and less than
or equal to 14 percent by mass.
[0038] Fe and Si
[0039] Iron (Fe) and silicon (Si) elements should be minimized in
amounts. Fe and Si precipitate as Al--Mg intermetallic compounds
typically containing Al--Mg--(Fe, Si); and other intermetallic
compounds than Al--Mg intermetallic compounds, such as Al--Fe and
Al--Si intermetallic compounds. The aluminum alloy sheet, if having
an Fe content of more than 1.0 percent by mass or a Si content of
more than 0.5 percent by mass, may suffer from precipitation of
such intermetallic compounds in excessively large amounts and
thereby have significantly deteriorated fracture toughness and
formability, resulting in significantly inferior stamping
performance. To avoid these, the aluminum alloy sheet has an Fe
content of 1.0 percent by mass or less and preferably 0.5 percent
by mass or less, and a Si content of 0.5 percent by mass or less
and preferably 0.3 percent by mass or less.
[0040] Ti, B, Mn, Cr, Zr, V, Cu, and Zn
[0041] Titanium (Ti) and boron (B) have the effect of allowing a
cast strip (ingot) to have a finer structure. Manganese (Mn),
chromium (Cr), zirconium (Zr), and vanadium (V) have the effect of
allowing a rolled sheet to have a finer structure; and copper (Cu)
and zinc (Zn) also have the effect of allowing a sheet to have
higher strengths. To exhibit these effects, one or more of these
elements may be contained within a range not adversely affecting
the stamping performance which features the alloy sheet according
to the present invention. Preferred contents of these elements to
be acceptable are 0.1 percent by mass or less of Ti, 0.05 percent
by mass or less of B, 0.3 percent by mass or less of Mn, 0.3
percent by mass or less of Cr, 0.3 percent by mass or less of Zr,
0.3 percent by mass or less of V, 1.0 percent by mass or less of
Cu, and 1.0 percent by mass or less Zn.
[0042] Next, detailed description will be made with reference to
FIG. 1 on a width-direction average Mg concentration Co,
thickness-direction average Mg concentrations Ci, and regional Mg
segregation degrees X of the aluminum alloy sheet, which regional
Mg segregation degrees X are defined by the width-direction average
Mg concentration Co and the thickness-direction average Mg
concentrations Ci.
[0043] Width-Direction Average Mg Concentration Co
[0044] With reference to FIG. 1(a), the width-direction average Mg
concentration Co may be determined in the following manner.
Initially, a square region each side of which has the dimension of
the entire sheet width W is set on a surface of an aluminum alloy
sheet 60. Plural width-direction measurement points Px are set at a
predetermined spacing "a" in a width direction and at a
predetermined spacing "b" in a length direction in the square
region, namely, a region surrounded by sides of the entire sheet
width W and sides of the sheet length L having the same dimension
with the entire sheet width W. Mg concentrations are measured at
the plural width-direction measurement points Px in the surface of
the aluminum alloy sheet 60. An average of the measured Mg
concentrations is defined as a width-direction average Mg
concentration Co which acts as an index for the level of Mg
segregation on the surface in the width direction of the aluminum
alloy sheet 60. The Mg concentrations may be measured by preparing
an electron probe X-ray microanalyzer (EPMA) capable of performing
linear analysis, and scanning the aluminum alloy sheet 60 in the
width direction using the EPMA.
[0045] Reproducibility in measurement of level of Mg segregation in
the width direction of the aluminum alloy sheet 60 may be obtained
by setting a square region, each side of which has the dimension of
the entire sheet width W, on a surface of the aluminum alloy sheet
60, and measuring Mg concentrations in the region on the surface of
the aluminum alloy sheet 60. The width-direction measurement points
Px are preferably set in the region in a number (number of points)
of 5 or more in the width direction excluding sheet edges and 5 or
more in the length direction, i.e., a total of 25 or more. The
spacing "a" in the width direction and the spacing "b" in the
length direction of the sheet may be set so as to set the
width-direction measurement points Px in a number of 25 or more. In
a more preferred embodiment, the spacing "b" in the length
direction may be set so as to be 0.5 to 2 times the spacing "a" in
the width direction.
[0046] Thickness-Direction Average Mg Concentration Ci
[0047] With reference to FIG. 1(b), the thickness-direction average
Mg concentrations Ci are calculated in the following manner.
Initially, plural thickness-direction measurement points Py are set
at a predetermined spacing "c" in a thickness direction throughout
the entire sheet thickness T. They are set for all the
width-direction measurement points Px in the region. Mg
concentrations (i.e., after-mentioned thickness-direction Mg
concentrations Ct) at corresponding sheet depth positions are
measured for each thickness-direction measurement point Py. An
average of the measured Mg concentrations is defined as a
thickness-direction average Mg concentration Ci which acts as an
index for level of Mg segregation in the thickness direction (depth
direction) of the aluminum alloy sheet 60. The Mg concentrations
may be measured with an EPMA as above. Specifically, Mg
concentrations at the respective thickness positions throughout the
entire sheet thickness T in the region may be measured by scanning,
in the thickness direction, cross sections of the aluminum alloy
sheet cut in the width direction.
[0048] The spacing "c" in the thickness direction of the sheet is
preferably set to 0.2 mm or less for ensuring reproducibility of
the level of Mg segregation in the thickness direction of the
aluminum alloy sheet 60. An initial measurement point of the
thickness-direction measurement points Py is the corresponding
width-direction measurement point Px at which a Mg concentration
has already been measured.
[0049] Regional Mg Segregation Degrees X
[0050] The regional Mg segregation degrees X are each defined as a
difference (Ci-Co) between each of the thickness-direction average
Mg concentrations (Ci) and the width-direction average Mg
concentration (Co), and act as an index for the level of Mg
segregation in the entire aluminum alloy sheet 60, namely, both in
the thickness direction and in the width direction. In the aluminum
alloy sheet 60 according to the present invention, absolute values
of the regional Mg segregation degrees X have a maximum of 0.5
percent by mass or less and an average of 0.1 percent by mass or
less. The width-direction average Mg concentration Co,
thickness-direction average Mg concentrations Ci, and regional Mg
segregation degrees X may be controlled by the chemical composition
of the aluminum alloy sheet 60 and after-mentioned manufacturing
conditions thereof, typified by cooling conditions upon casting,
thickness or facing amount (stock removal) of a cast strip, soaking
conditions, and final annealing conditions.
[0051] If the regional Mg segregation degrees X have a large
positive maximum and/or a large positive average, .beta. phases are
liable to precipitate due to Mg segregation. Such .beta. phases
will cause fracture and, when increased, adversely affect strengths
and elongation of the aluminum alloy sheet, resulting in
insufficient formability. If the regional Mg segregation degrees X
have a large negative maximum and/or a large negative average, the
aluminum alloy sheet may include a large number of local areas with
significantly low Mg concentrations. The areas with significantly
low Mg concentrations have low strengths. Therefore only the areas
with low Mg concentrations preferentially deform upon tensile
deformation in forming, resulting in nonuniform deformation. This
may lead to local or partial concentration of strain upon forming
and cause the aluminum alloy sheet to have insufficient formability
particularly due to inferior elongation.
[0052] For these reasons, an aluminum alloy sheet 60 may have
insufficient formability if having a maximum of more than 0.5
percent by mass and/or an average of more than 0.1 percent by mass
in absolute values of regional Mg segregation degrees X, i.e., if
not satisfying either one or both of the conditions specified in
the present invention.
[0053] The aluminum alloy sheet 60 according to the present
invention is preferably controlled on, in addition to the regional
Mg segregation degrees X, thickness-direction Mg segregation
degrees Y to predetermined levels or less. The thickness-direction
Mg segregation degrees Y are each defined by a thickness-direction
Mg concentration Ct and a thickness-direction average Mg
concentration Ci.
[0054] Thickness-Direction Mg Concentrations Ct and
Thickness-Direction Average Mg Concentration Ci
[0055] The thickness-direction Mg concentrations Ct are Mg
concentrations measured at the plural thickness-direction
measurement points Py illustrated in FIG. 1(b), as described above.
The thickness-direction average Mg concentration Ci is an average
of the measured thickness-direction Mg concentrations Ct.
[0056] The thickness-direction Mg concentrations Ct and the
thickness-direction average Mg concentration Ci are Mg
concentrations in the thickness direction of the sheet measured for
at least one width-direction measurement point Px in the region, as
measured upon determination of the regional Mg segregation degrees
X. The thickness-direction Mg concentrations Ct are preferably
determined by measuring Mg concentrations for one width-direction
measurement point Px in the center part of sheet width; and are
more preferably determined by measuring Mg concentrations for three
width-direction measurement points Px including one in the center
part and two in the vicinities of both ends of sheet width, and
averaging the measured values.
[0057] Thickness-Direction Mg Segregation Degrees Y
[0058] The thickness-direction Mg segregation degrees Y are each
defined as a difference (Ct-Ci) between a thickness-direction Mg
concentration (Ct) and the thickness-direction average Mg
concentration (Q), and act as an index for the level of Mg
segregation in the thickness direction of the aluminum alloy sheet
60. The thickness-direction Mg segregation degrees Y, when used in
combination with the regional Mg segregation degrees X, can
satisfactorily reproduce the level of Mg segregation throughout the
entire aluminum alloy sheet 60. In the aluminum alloy sheet 60
according to the present invention, absolute values of the
thickness-direction Mg segregation degrees Y preferably have a
maximum of 4 percent by mass or less and an average of 0.8 percent
by mass or less. The thickness-direction Mg concentrations Ct,
thickness-direction average Mg concentrations Ci, and
thickness-direction Mg segregation degrees Y may be controlled by
the chemical composition of the aluminum alloy sheet 60 and the
manufacturing conditions thereof, typified by cooling conditions
upon casting, thickness or facing amount (stock removal) of the
cast strip, soaking conditions, and final annealing conditions.
[0059] If the thickness-direction Mg segregation degrees Y have a
large positive maximum and/or a large positive average, .beta.
phases are liable to precipitate due to Mg segregation. Such .beta.
phases will cause fracture and, when increased, adversely affect
strengths and elongation of the aluminum alloy sheet, resulting in
insufficient formability. If the thickness-direction Mg segregation
degrees Y have a large negative maximum and/or a large negative
average, the aluminum alloy sheet may include a large number of
local areas with significantly low Mg concentrations. The areas
with significantly low Mg concentrations have low strengths.
Therefore, only the areas with low Mg concentrations preferentially
deform upon tensile deformation in forming, resulting in nonuniform
deformation. This may lead to local or partial concentration of
strain upon forming and cause the aluminum alloy sheet to have
insufficient formability particularly due to inferior
elongation.
[0060] For these reasons, an aluminum alloy sheet 60 may exhibit
insufficient formability if absolute values of the
thickness-direction Mg segregation degrees Y have a maximum of more
than 4 percent by mass and/or an average of more than 0.8 percent
by mass. Specifically, an aluminum alloy sheet 60 not satisfying
either one or both of the specific conditions may exhibit
insufficient formability.
[0061] Average Grain Size
[0062] The aluminum alloy sheet according to the present invention
preferably has an average grain size of 100 .mu.m or less in its
surface.
[0063] The aluminum alloy sheet, when including fine grains with an
average grain size of 100 .mu.m or less in its surface, may have
better stamping performance. The aluminum alloy sheet, if including
coarse grains with an average grain size of more than 100 .mu.m,
may be liable to have insufficient stamping performance, resulting
in defects such as cracking and orange peel surfaces upon forming.
In contrast, the aluminum alloy sheet, if including excessively
fine grains with an excessively small average grain size, may
suffer from stretcher strain (SS) marks upon stamping, which marks
are peculiar to 5xxx aluminum alloy sheets. To avoid this, the
aluminum alloy sheet preferably has an average grain size of 20
.mu.m or more.
[0064] As used herein the term "grain size" refers to a greatest
dimension of a grain in the length direction. The grain size is
measured in the L direction according to a line intercept method by
mechanically polishing the aluminum alloy sheet to 0.05 to 0.1 mm,
electrolytically etching the surface of the polished sheet, and
observing the etched surface with an optical microscope at a
100-fold magnification. In this measurement, one measurement line
is set to have a length of 0.95 mm. Three lines are set per one
field of view, and a total of five fields of view are observed.
Thus, the total measurement line length is 0.95 mm times 15 mm.
[0065] Next, a method for manufacturing the aluminum alloy sheet
will be illustrated.
[0066] The aluminum alloy sheet according to the present invention
may be manufactured through a melting-casting step, a soaking step,
a cold rolling step, and a final annealing step. The respective
steps will be illustrated below.
{Melting-Casting Step}
[0067] The melting-casting step is a step of melting a
high-Mg-content Al--Mg alloy having the aforementioned chemical
composition to give a molten metal, and preparing a cast strip from
the molten metal by a continuous strip casting process. The
continuous strip casting process is preferably a continuous casting
process using a fixed graphite mold.
[0068] The continuous casting process using a fixed graphite mold
is performed with continuous strip casting equipment 10 as
illustrated in FIG. 2. Initially, a molten metal 2 stored in a
holding furnace 1 is poured through an inlet la into a continuous
mold 3 (fixed graphite mold 4). Next, the molten metal 2 is
solidified in the fixed graphite mold 4 which is cooled with a
water-cooling jacket 5. This gives a cast strip 6 having a small
thickness. The prepared cast strip 6 is carried out by rolls 7 to a
subsequent step. This casting process employs cooling at a higher
rate than that in the DC casting process and thereby provides a
finer cast structure and contributes to better stamping
performance. In addition, this casting process can give a cast
strip having a relatively small thickness of about 5 mm and
eliminates the need of hot rough rolling, hot finish rolling, and
other steps performed after casting on customary DC ingots having
thicknesses of 200 to 600 mm.
[0069] Cooling Rate
[0070] In the continuous casting process using a fixed graphite
mold, cooling may be performed at a rate of 15.degree. C./s
provided that the cast strip 6 has a thickness of from 5 to 20 mm.
Cooling, if performed at an excessively low rate, may cause the
aluminum alloy sheet to suffer from high level of Mg segregation,
and this may impede the control of Mg segregation degrees within
the ranges specified in the present invention to fail to suppress
precipitation of .beta. phases induced by Mg segregation. In this
connection, the regional Mg segregation degrees X and
thickness-direction Mg segregation degrees Y are hereinafter also
synthetically referred to as "Mg segregation degrees." In addition,
.beta. phases are generally liable to be coarse and precipitate in
large amounts. This may highly possibly cause the aluminum alloy
sheet to have significantly inferior stamping performance.
[0071] It is difficult to measure the cooling rate directly. The
cooling rate may therefore be determined from a dendrite arm
spacing (secondary dendrite arm spacing: DAS) of the cast strip 6
after casting by a known method, which is typified by a method
described in The Japan Institute of Light Metals (ed.): "Measuring
method of dendrite arm spacing and cooling rate of aluminum alloy,"
issued on Aug. 20, 1988. Specifically, the cooling rate may be
determined by measuring an average spacing "d" of adjacent
secondary dendrite arms (secondary arms) in the cast structure of
the cast strip 6 according to the line intercept method (in three
or more fields of view at ten or more intercepting points). The
cooling rate is then determined according to the following
expression: d=62.times.C-0.337, wherein d represents the secondary
dendrite arm spacing (mm); and C represents the cooling rate
(.degree. C./s). The cooling rate can therefore also be said as a
solidification rate.
[0072] Pouring Temperature
[0073] The continuous casting process using a fixed graphite mold
may employ pouring of the molten metal 2 into the fixed graphite
mold 4 at a temperature higher than the liquidus temperature by
50.degree. C. or more and 250.degree. C. or less and preferably at
a temperature higher than the liquidus temperature by 100.degree.
C. or more and 150.degree. C. or less. Pouring, if performed at a
temperature below the temperature higher than the liquidus
temperature by 50.degree. C. (liquidus temperature+50.degree. C.,
may cause solidification of the molten metal within the mold to
often cause rupture of the cast strip. Pouring, if performed at a
temperature above the temperature of higher than the liquidus
temperature by 250.degree. C. (liquidus temperature+250.degree.
C.), may cause cooling upon casting to proceed slowly at a low rate
and may thereby cause higher level of Mg segregation. This may
impede the control of the Mg segregation degrees within the ranges
specified in the present invention and may impede suppression of
the precipitation of .beta. phases and deterioration in formability
due to large Mg segregation degrees.
[0074] Withdrawing Method
[0075] In the continuous casting process using a fixed graphite
mold, the cast strip 6 may be backed by periodically rotating the
rolls 7 in a direction opposite to the casting direction, for the
stabilization of casting. The rolls 7 transport the cast strip 6 in
the casting direction in a forward operation The backing may be
performed at a stroke length of 0.5 mm or more and 5 mm or less,
and preferably 1 mm or more and 3 mm or less. The castability is
more stabilized when the cast strip is held for a holding time of 1
second or shorter before the backing.
[0076] If backing is performed at a stroke length of more than 5
mm, a segregated layer with a high Mg concentration generated in a
surface of the cast strip 6 may penetrate into the strip to cause
cracking of the cast strip at the site to thereby cause rupture. If
backing is performed at a stroke length of less than 0.5 mm, a
solid-liquid coexisting zone may not be compressed and may remain
as susceptible to rupture, and this may cause the cast strip 6 to
rupture in a region including the solid-liquid coexisting zone. To
avoid these, the backing may be performed at a stroke length of
from 0.5 mm or more and 5 mm or less.
[0077] Average Casting Rate
[0078] Casting of the molten metal 2 in the fixed graphite mold 4
in the continuous casting process using the fixed graphite mold may
be performed at an average casting rate of 100 mm/min or more and
500 mm/min or less and preferably 250 mm/min or more and 350 mm/min
or less. Casting, if performed at an average casting rate of less
than 100 mm/min, may cause rapid solidification of the molten metal
2 in the vicinity of the inlet 1a to increase the withdrawal
resistance of the portion upon withdrawing by the rolls 7, and the
resulting cast strip 6 may be susceptible to rupture. Casting, if
performed at an average casting rate of more than 500 mm/min, may
cause molten metal leakage in the vicinity of a cast strip outlet
4a due to insufficient cooling.
[0079] Cast Strip Thickness
[0080] The cast strip 6 which has been continuously cast according
to the continuous casting process using a fixed graphite mold may
have a thickness in the range of 5 mm or more and 20 mm or less. If
a cast strip 6 having a thickness of less than 5 mm is to be
formed, the molten metal 2 may rapidly solidify in the vicinity of
the inlet 1a to increase the withdrawal resistance in the region
upon withdrawing by the rolls 7, and the resulting cast strip 6 may
be susceptible to rupture. If a cast strip 6 having a thickness of
more than 20 mm is to be formed, cooling in the casting may be
performed very slowly at an extremely low cooling rate to increase
the level of Mg segregation. This may impede the control of Mg
segregation degrees within the ranges specified in the present
invention and impede suppression of precipitation of .beta. phases
induced by large Mg segregation degrees. In addition, .beta. phases
are generally liable to be coarse and precipitate in large amounts.
This may highly possibly cause the aluminum alloy sheet to have
significantly inferior stamping performance.
[0081] Facing
[0082] The continuous casting process using a fixed graphite mold
preferably employs facing in which both sides of the prepared cast
strip 6 are cut or shaved by a predetermined amount, because Mg
segregation is liable to occur in the surfaces of the cast strip 6.
The facing removes Mg-segregated regions on both sides of the strip
and can thereby control the Mg segregation degrees within the
ranges specified in the present invention. A depth of the
Mg-segregated region corresponds to the back stroke length, and the
facing may be performed in an amount corresponding to the back
stroke length in the withdrawing.
[0083] The continuous strip casting process has been illustrated
above by taking the continuous casting process using a fixed
graphite mold as an example, but the process is not limited
thereto. The continuous strip casting process may be any process,
as long as capable of controlling the Mg segregation degrees of the
aluminum alloy sheet within ranges specified in the present
invention and may be, for example, a twin-roll continuous casting
process.
[0084] The twin-roll continuous casting process may be performed
with continuous strip casting equipment 100 as illustrated in FIG.
3. Initially a molten metal 300 is poured from a holding furnace
200 through a molten-metal-feeding nozzle 400 into a roll bite
between a pair of rotating water-cooled copper molds (twin rolls
500) and solidified. The solidified metal immediately after
solidification is rolled and quenched in the roll bite between the
twin rolls 500 and thereby yields a cast strip 600 having a small
thickness. The twin-roll continuous casting process is exemplified
by a Hunter process and a continuous casting between cylinders (3C)
process. The twin-roll continuous casting process can give a sheet
(strip) having a relatively small thickness of 1 to 13 mm and
eliminates the need for steps, such as hot rough rolling and hot
finish rolling after casting, performed in manufacture of customary
direct chill (DC) ingots (having thicknesses of 200 to 600 mm).
{Soaking Step}
[0085] The soaking step is a step of subjecting the cast strip 6
prepared in the preceding step to a predetermined soaking. The
soaking is performed at a temperature of 400.degree. C. or above
and the liquidus temperature or below for a necessary duration.
When the cast strip 6 prepared by the continuous strip casting
process is soaked in a continuous heat-treating furnace, the heat
treatment (soaking) is performed for a duration of roughly about
one second (1 s) or shorter. The soaking reduces the level of Mg
segregation and allows the aluminum alloy sheet to have Mg
segregation degrees controlled within the ranges specified in the
present invention.
[0086] There is a sufficient possibility of the generation of
Al--Mg intermetallic compounds (.beta. phases) both in temperature
rise and cooling of the cast strip 6 in the soaking, if the rate of
temperature rise and/or the cooling rate is excessively low. Such
.beta. phases are highly possibly generated at temperatures of the
cast strip central part of from 200.degree. C. to 400.degree. C.
during temperature rise; and at temperatures of from the soaking
temperature down to 100.degree. C. during cooling. To suppress the
generation of .beta. phases, heating up to the soaking temperature
is preferably performed at an average rate of temperature rise of
5.degree. C./s or more at temperatures of the cast strip central
part of from 200.degree. C. to 400.degree. C.; and cooling down
from the soaking temperature is preferably performed at an average
cooling rate of 5.degree. C./s or more at temperatures of the cast
strip central part of from the soaking temperature down to
100.degree. C.
{Cold Rolling Step}
[0087] The cold rolling step is a step of cold-rolling the soaked
cast strip 6 to a thickness of a product sheet typically of 0.1 mm
or more and 13 mm or less. The cold rolling converts the cast
structure into a deformation structure. Accordingly, process
annealing is preferably performed midway through the cold rolling
so as to provide a cold rolling reduction in final cold rolling of
60% or less, when the cast strip 6 before cold rolling has a large
thickness. The degree of conversion into a deformation structure as
a result of cold rolling may vary also depending on the cold
rolling reduction in cold rolling. For this reason, the cast
structure may remain for the control of the metallic texture, but
such residual cast structure is acceptable within a range not
adversely affecting formability and mechanical properties.
{Final Annealing Step}
[0088] The final annealing step is a step of subjecting the
cold-rolled sheet prepared in the preceding step to a predetermined
final annealing. The final annealing step performs final annealing
on the cold-rolled sheet at a temperature of 400.degree. C. or
above and below the liquidus temperature (.degree. C.). The final
annealing reduces the level of Mg segregation, allows the aluminum
alloy sheet to have Mg segregation degrees within ranges specified
in the present invention, and protects the aluminum alloy sheet
from undergoing precipitation of .beta. phases and from having
insufficient stamping performance each due to Mg segregation.
[0089] A final annealing performed at a temperature of below
400.degree. C. may highly possibly fail to provide solutionizing
effects and may fail to reduce the level of Mg segregation
effectively. To avoid these, the final annealing is preferably
performed at a temperature of 450.degree. C. or above. The sheet
after the final annealing is preferably cooled at an average
cooling rate as high as possible of 10.degree. C./s or more at
temperatures of from 500.degree. C. down to 300.degree. C. Cooling
after the final annealing, if performed at a low average cooling
rate of less than 10.degree. C./s, may contrarily increase the
level of Mg segregation during the cooling process. In this case,
the resulting aluminum alloy sheet may fail to have Mg segregation
degrees controlled within the ranges specified in the present
invention and may possibly suffer from the precipitation of .beta.
phases and decrease in stamping performance due to large Mg
segregation degrees. The average cooling rate is preferably
15.degree. C./s or more.
EXAMPLES
[0090] Next, some working examples according to the present
invention will be illustrated below.
[0091] Molten metals of Al--Mg alloys having chemical compositions
given in Table 1 (Examples A, B, C, D, and E and Comparative
Examples F and G) were cast under conditions given in Table 2 to
give cast strips having thicknesses given in Table 2. The casting
was performed by the continuous casting process using a fixed
graphite mold or the twin-roll continuous casting process as
mentioned above. The respective cast strips were selectively
subjected to facing and soaking under conditions given in Table 2
and then cold-rolled to give cold-rolled sheets having a thickness
of 1.0 mm or 11.0 mm without hot rolling. No process annealing was
performed during the cold rolling. Next, the respective cold-rolled
sheets were subjected to final annealing in a continuous annealing
furnace at temperatures and cooling conditions given in Table 2 for
a holding time at the annealing temperature of one second or
shorter and thereby yielded aluminum alloy sheets for forming as
Examples Nos. 1 to 5 and Comparative Examples Nos. 6 to 20. The
aluminum alloy sheet for forming as Comparative Example No. 6 was
prepared by a method through the twin-roll continuous casting
process described in PTL 3.
[0092] The continuous casting process using a fixed graphite mold
was performed at a back stroke length of 3 mm, an average casting
rate of 300 mm/min, and a casting temperature (pouring temperature)
of higher than the liquidus temperature by 140.degree. C. The
twin-roll continuous casting process was performed at a
circumferential speed of the twin rolls of 70 m/min and at a
pouring temperature for pouring the molten metal into a roll bite
between the twin rolls of higher than the liquidus temperature by
20.degree. C. This process was performed without lubrication of the
surfaces of the twin rolls.
[0093] The liquidus temperatures of the respective alloys were
calculated with a software for thermodynamic calculation
Thermo-Calc Ver. R (Al-DATA Ver. 6).
[0094] The resulting formable aluminum alloy sheets (Examples Nos.
1 to 5 and Comparative Examples Nos. 6 to 22) were subjected to
calculation and evaluation on regional Mg segregation degrees X and
thickness-direction Mg segregation degrees Y according to the
following procedures. The results are indicated in Table 2.
[0095] FIG. 4 depicts the calculated regional Mg segregation
degrees X of Example No. 1; and FIG. 5 depicts the calculated
regional Mg segregation degrees X of Comparative Example No. 6.
FIG. 6 depicts the calculated thickness-direction Mg segregation
degrees Y of Example 1; and FIG. 7 depicts the calculated
thickness-direction Mg segregation degrees Y of Comparative Example
No. 16.
[0096] Calculation and Evaluation of Regional Mg Segregation
Degrees X
[0097] Initially, a square region with a dimension of one side of
100 mm was set on a surface of a sample formable aluminum alloy
sheet. Next, five points excluding sheet edges were set in the
width direction at a spacing of 16.6 mm (spacing "a"), and five
points were set in the length direction at a spacing of 25 mm
(spacing "b"), each in the region. Thus, a total of 25
width-direction measurement points Px (Nos. 1 to 25) was set (see
FIG. 1(a)). Mg concentrations in the surface of the aluminum alloy
sheet were measured at the respective measurement points and
averaged to give a width-direction average Mg concentration Co.
[0098] Next, plural thickness-direction measurement points Py were
set in the thickness direction at a spacing of 0.01 mm (spacing
"c") for each of the width-direction measurement points Px (Nos. 1
to 25) (see FIG. 1(b)). Mg concentrations of the aluminum alloy
sheet at the respective measurement points (predetermined thickness
positions (predetermined depth positions)) were measured and
averaged to give a thickness-direction average Mg concentration
Ci.
[0099] For each of the width-direction measurement points Px (Nos.
1 to 25), a regional Mg segregation degree X was calculated from
the thickness-direction average Mg concentration Ci and the
width-direction average Mg concentration Co, which regional Mg
segregation degree X was defined as a difference between them
(Ci-Co) (see FIGS. 4 and 5). The Mg concentrations were measured
with an EPMA (JXA-8800RL, electron probe X-ray microanalyzer
supplied by JEOL Ltd).
[0100] The regional Mg segregation degrees X were evaluated in the
following manner. A sample having a maximum of absolute values of
regional Mg segregation degrees X of 0.5 percent by mass or less
was evaluated as satisfactory (.largecircle.); and a sample having
the maximum of more than 0.5 percent by mass was evaluated as
unsatisfactory (x). A sample having an average of absolute values
of regional Mg segregation degrees X of 0.1 percent by mass or less
was evaluated as satisfactory (.largecircle.); and a sample having
the average of more than 0.1 percent by mass was evaluated as
unsatisfactory (x).
[0101] Calculation and Evaluation of Thickness-Direction Mg
Segregation Degrees Y
[0102] One (No. 13) of the width-direction measurement points (Nos.
1 to 25) was selected, and Mg concentrations measured at that point
(No. 13) in the thickness direction (at the plural
thickness-direction measurement points Py) were defined as
thickness-direction Mg concentrations Ct. Thickness-direction Mg
segregation degrees Y were calculated each as a difference (Ct-Ci)
between each of the thickness-direction Mg concentrations Ct and
the thickness-direction average Mg concentration Ci which had been
calculated as an average of Mg concentrations measured at the
respective (thickness-direction) measurement points.
Thickness-direction measurement points Py, when positioned at a
depth of 0.01 mm or 1.0 mm, are positioned in surfaces of the alloy
sheet (see FIGS. 6 and 7).
[0103] The thickness-direction Mg segregation degrees Y were
evaluated in the following manner. A sample having a maximum of
absolute values of thickness-direction Mg segregation degrees Y of
4 percent by mass or less was evaluated as satisfactory
(.largecircle.); and a sample having the maximum of more than 4
percent by mass was evaluated as unsatisfactory (x). A sample
having an average of absolute values of thickness-direction Mg
segregation degrees Y of 0.8 percent by mass or less was evaluated
as satisfactory (.largecircle.); and a sample having the average of
more than 0.8 percent by mass was evaluated as unsatisfactory
(x).
[0104] Average Grain Size
[0105] Average grains sizes of the prepared formable aluminum alloy
sheets (Examples Nos. 1 to 5 and Comparative Examples 6 to 22) were
measured according to the aforementioned measuring method.
[0106] Examples Nos. 1 to 5 and Comparative Examples Nos. 6 to 10,
12 to 17, and 19 to 22 had average grain sizes in the range of 30
to 60 .mu.m. Comparative Examples Nos. 11 and 18 had average grain
sizes of more than 100 .mu.m.
[0107] Evaluation of Stamping Performance
[0108] Stamping performances of the prepared formable aluminum
alloy sheets (Examples Nos. 1 to 5 and Comparative Examples Nos. 6
to 22) were evaluated according to the following procedure. The
results are indicated in Table 2.
[0109] Specimens were sampled from the alloy sheets and subjected
to tensile tests to measure a tensile strength (TS (MPa)) and a
total elongation (EL ( %)). The stamping performance was evaluated
based on a strength-ductility balance defined as the product of TS
and EL (TS.times.EL). A sample having a strength-ductility balance
of 11000 or more was evaluated as accepted (.largecircle.); and a
sample having a strength-ductility balance of less than 11000 was
evaluated as rejected (x).
[0110] The specimens were sampled from each alloy sheet at
arbitrary five points arranged across the longitudinal direction at
a spacing of 100 mm or more. The measured tensile strengths and
elongations of the five specimens per sample were respectively
averaged to give a TS and an EL of the sample. The tensile tests
were performed according to JIS Z 2201 using JIS No. 5 specimens.
The specimens were prepared so that their longitudinal directions
correspond to the rolling direction of the sample alloy sheet. The
tensile tests were performed at a constant crosshead speed of 5
mm/min until the specimen was ruptured.
TABLE-US-00001 TABLE 1 Chemical composition (mass percent) Liquidus
Category Code Mg Fe Si Ti B Mn Cr Zr V Cu Zn temperature (.degree.
C.) Examples A 10.0 0.25 0.21 0.01 0.002 -- -- -- -- -- -- 608 B
10.0 0.25 0.21 0.01 0.002 0.20 0.20 0.20 -- 0.80 0.20 605 C 6.1
0.25 0.21 0.01 0.002 -- -- -- -- -- -- 628 D 8.1 0.25 0.21 0.01
0.002 -- -- -- -- -- -- 618 E 14.8 0.25 0.21 0.01 0.002 -- -- -- --
-- -- 582 Comparative F 5.8 0.25 0.21 0.01 0.002 -- -- -- -- -- --
630 Examples G 15.3 0.25 0.21 0.01 0.002 -- -- -- -- -- -- 579
TABLE-US-00002 TABLE 2 Casting conditions Facing Cold rolling
Cooling Cast strip Sheet Sheet Casting rate thickness thickness
thickness Category Number Alloy process (.degree. C./s) (mm) (mm)
Soaking (mm) Examples 1 A graphite mold 400 10 4 450.degree. C. for
1 s or shorter 1.0 2 B graphite mold 400 10 4 490.degree. C. for 1
s or shorter 1.0 3 C graphite mold 400 10 4 400.degree. C. for 1 s
or shorter 1.0 4 D graphite mold 400 10 4 430.degree. C. for 1 s or
shorter 1.0 5 E graphite mold 400 10 4 460.degree. C. for 1 s or
shorter 1.0 Comparative 6 A twin-roll 400 5.5 -- 450.degree. C. for
1 s or shorter 1.0 Examples 7 F twin-roll 800 1.7 -- 450.degree. C.
for 1 s or shorter 1.0 8 G twin-roll 800 1.7 -- 450.degree. C. for
1 s or shorter 1.0 9 A twin-roll 800 1.7 -- -- 1.0 10 B twin-roll
800 1.7 -- -- 1.0 11 A twin-roll 50 10 -- 450.degree. C. for 1 s or
shorter 1.0 12 A twin-roll 800 1.7 -- 450.degree. C. for 1 s or
shorter 1.0 13 A twin-roll 800 1.7 -- 450.degree. C. for 1 s or
shorter 1.0 14 F graphite mold 400 10 4 450.degree. C. for 1 s or
shorter 1.0 15 G graphite mold 400 10 4 450.degree. C. for 1 s or
shorter 1.0 16 A graphite mold 400 10 4 -- 1.0 17 B graphite mold
400 10 4 -- 1.0 18 A graphite mold 5 25 19 450.degree. C. for 1 s
or shorter 11.0 19 A graphite mold 400 10 4 450.degree. C. for 1 s
or shorter 1.0 20 A graphite mold 400 10 4 450.degree. C. for 1 s
or shorter 1.0 21 A twin-roll 400 5.5 2 450.degree. C. for 1 s or
shorter 1.0 22 A graphite mold 400 10 -- 450.degree. C. for 1 s or
shorter 1.0 Thickness-direction Mg Regional Mg Final annealing
segregation degree segregation degree Stamping Cooling Maximum
Average Maximum Average performance Temperature rate (mass (mass
(mass (mass TSxEL Category Number (.degree. C.) (.degree. C./s)
percent) percent) percent) percent) (MPa %) Examples 1 450 20.0
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.smallcircle. 2 450 20.0 .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. 3 450 20.0 .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. 4 450 20.0 .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle. 5 450 20.0
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.smallcircle. Comparative 6 450 20.0 .smallcircle. .smallcircle. x
x x Examples 7 450 20.0 .smallcircle. .smallcircle. .smallcircle.
.smallcircle. x 8 450 20.0 x x x x x 9 450 20.0 x x x x x 10 450
20.0 x x x x x 11 450 20.0 x x x x x 12 350 20.0 x x x x x 13 450
5.0 x x x x x 14 450 20.0 .smallcircle. .smallcircle. .smallcircle.
.smallcircle. x 15 450 20.0 x x x x x 16 450 20.0 x x x x x 17 450
20.0 x x x x x 18 450 20.0 x x x x x 19 350 20.0 x x x x x 20 450
5.0 x x x x x 21 450 20.0 .smallcircle. .smallcircle. x x x 22 450
20.0 x x x x x
[0111] The results in Tables 1 and 2 demonstrate that Examples Nos.
1 to 5 satisfying the conditions specified in the present invention
were superior in stamping performance to Comparative Examples Nos.
6 to 22 not satisfying the conditions in the present invention.
[0112] Specifically, Comparative Example No. 6, the alloy sheet
described in PTL 3, failed to control Mg segregation degrees
(regional Mg segregation degrees) within the ranges specified in
the present invention and had poor stamping performance.
Comparative Examples Nos. 7 and 14 had Mg segregation degrees
controlled within the ranges specified in the present invention,
but had Mg contents less than the lower limit, thereby had a poor
strength-ductility balance and exhibited poor stamping performance.
Comparative Examples Nos. 8 and 15 had Mg contents more than the
upper limit, thereby had large Mg segregation degrees, and
exhibited poor stamping performance. Comparative Examples Nos. 9,
10, 16, and 17 did not undergo soaking, thereby had large Mg
segregation degrees, and exhibited poor stamping performance.
Comparative Examples Nos. 11 and 18 underwent cooling in the
casting performed at a low cooling rate, thereby had large Mg
segregation degrees, and exhibited poor stamping performance.
Comparative Examples Nos. 12 and 19 underwent final annealing
performed at a low temperature, thereby had large Mg segregation
degrees, and exhibited poor stamping performance. Comparative
Examples 13 and 20 underwent final annealing performed at a low
cooling rate, thereby had large Mg segregation degrees, and
exhibited poor stamping performance. Comparative Example 21
underwent facing of both sides of a cast strip by 1.75 mm, which
cast strip had been prepared by the twin-roll continuous casting
process. This comparative example failed to have Mg segregation
degrees (regional Mg segregation degrees) controlled within the
ranges specified in the present invention and exhibited poor
stamping performance. Comparative Example 22 did not undergo facing
of both sides of a cast strip prepared by the continuous casting
process using a fixed graphite mold, thereby had large Mg
segregation degrees, and exhibited poor stamping performance.
[0113] While the present invention has been described with
reference to embodiments and examples thereof, it will be
understood that the invention is not limited to such specific
embodiments thereof; and various modifications and changes may be
made therein without departing from the spirit and scope of the
invention as hereinafter claimed. The present application is based
on Japanese Patent Application No. 2010-187756 filed on Aug. 25,
2010, the entire contents of which are incorporated herein by
reference.
REFERENCE SIGNS LIST
[0114] a, b, c spacing
[0115] L sheet length
[0116] W sheet width
[0117] T sheet thickness
[0118] Px width-direction measurement point
[0119] Py thickness-direction measurement point
[0120] 1 holding furnace
[0121] 1a inlet
[0122] 2 molten metal
[0123] 3 continuous casting mold
[0124] 4 fixed graphite mold
[0125] 4a cast strip outlet
[0126] 5 water-cooling jacket
[0127] 6 cast strip
[0128] 7 roll
[0129] 10 continuous strip casting equipment
[0130] 100 continuous strip casting equipment
[0131] 200 holding furnace
[0132] 300 molten metal
[0133] 400 molten-metal-feeding nozzle
[0134] 500 twin roll
[0135] 600 cast strip
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